The invention concerns a method for detecting the opening of a passive pressure control valve, which conducts fuel from a common rail system back to a fuel tank.
In a common rail system, a high-pressure pump pumps the fuel from a fuel tank into a rail. The admission cross section to the high-pressure pump is determined by a variable suction throttle. Injectors are connected to the rail. They inject the fuel into the combustion chambers of the internal combustion engine. Since the quality of the combustion is decisively determined by the pressure level in the rail, this pressure is automatically controlled. The closed-loop high pressure control system comprises a pressure controller, the suction throttle with the high-pressure pump, and the rail as the controlled system. In this closed-loop high pressure control system, the controlled variable is the pressure level in the rail. The measured pressure values in the rail are converted by a filter to an actual rail pressure and compared with a set rail pressure. The control deviation obtained by this comparison is converted to a control signal for the suction throttle by a pressure controller, for example, with PIDT1 response. The control signal corresponds to a volume flow in the unit liters/minute. The control signal is typically electrically generated as a PWM signal (pulse-width-modulated signal). The I component of the pressure controller and the actuating variables, e.g., the PWM signal for acting on the throttle valve, which are derived from the correcting variable, will be referred to as characteristics of the closed-loop control system in the remainder of the text. The closed-loop high pressure control system described above is disclosed in DE 103 30 466 B3.
To protect against an excessively high pressure level, a passive pressure control valve is installed in the rail. If the pressure level exceeds a preset value, the pressure control valve opens to conduct fuel from the rail back to the fuel tank.
The following problem can arise under practical conditions: a load reduction is immediately followed by an increase in engine speed. At a constant set speed, an increasing engine speed causes an increase in the magnitude of the speed control deviation. A speed controller responds to this by reducing the injection quantity as a correcting variable. A smaller injection quantity in turn causes less fuel to be taken from the rail, so that there is a rapid increase in the pressure level in the rail. The situation is further complicated by the fact that the output of the high-pressure pump depends on the engine speed. An increasing engine speed means a higher pump output, and this produces a further increase in pressure in the rail. Since the high pressure control system has a relatively long response time, the rail pressure can continue to rise until the pressure control valve opens, e.g., at 1,950 bars. This causes the rail pressure to drop very rapidly to a value of about 800 bars. At this pressure level, an equilibrium state develops between fuel pumped in and fuel removed. This means that despite the opened pressure control valve, the rail pressure does not drop further. As a result of the pressure loss, the efficiency of the internal combustion engine is reduced, and clearly visible clouding of the exhaust gas occurs.
German Patent Application with the official file number DE 10 2006 040 441.6, for which a prior printed publication has not yet appeared, proposes a method in which, after a load reduction, opening of the passive pressure control valve is detected when the rail pressure exceeds a first limit and a second limit. As an alternative to this, it is provided that opening of the pressure control valve is detected after the first limit if a strongly negative pressure gradient develops or if an impermissible control deviation or correcting variable arises. In practice it has been found that this method is not yet optimum for all operating points.